GPCR signaling along the endocytic pathway
Introduction
Agonist-induced endocytosis of GPCRs was initially recognized as a phenomenon coinciding with rapid desensitization of G protein-mediated cellular responses [1, 2]. The discovery that arrestins (also called β-arrestins) can promote GPCR endocytosis as well as desensitization provided a mechanistic link between these phenomena (reviewed in [3]). The identification of later molecular sorting operations that govern whether endocytosis facilitates reversal of the desensitized state (resensitization) or persistent attenuation (down-regulation) of cellular responsiveness further solidified a close relationship between endocytosis and reduced GPCR signaling (reviewed in [4]) (Figure 1). The present article discusses accumulating evidence suggesting that the endocytic pathway also functions, conversely, to promote receptor-mediated signaling responses. We will discuss G protein-independent signaling mechanisms only briefly, due to limited space and because authoritative reviews discussing aspects of this topic have appeared recently (e.g. [5, 6]). We focus here on roles of the endocytic pathway in promoting cellular signaling mediated by heterotrimeric G proteins.
Section snippets
G protein-independent signaling from endosomes
The idea that endosomes might be sites of receptor-mediated signaling emerged from studies of growth factor receptors, in which subcellular fractionation (and later live cell imaging) experiments detected tyrosine-phosphorylated receptors, together with signaling adaptors and associated kinases, in endosomes (reviewed in [7, 8]). The discovery that arrestins, like traditional adaptor proteins involved in growth factor signaling, bind various kinases in addition to receptors motivated the
Evidence for G protein-dependent signaling from intracellular membranes
Heterotrimeric G proteins have long been recognized at intracellular membrane locations as well as at the plasma membrane (reviewed in [22, 23]), but it is largely unknown if internal heterotrimeric G proteins are substrates for activation by GPCRs. Because G protein subunits present on internal membranes can themselves redistribute and functionally interact with membrane trafficking machineries, the presence of activated G proteins on internal membranes would not necessarily require their
Sustained GPCR-G protein signaling from endosomes
Evidence for GPCR-G protein signaling from endosomes in mammalian cells emerged from study of the delayed therapeutic effects of an immunomodulatory drug used in the treatment of multiple sclerosis, FTY720 (fingolimod). FTY720 is an agonist of sphingosine 1-phosphate (S1P) receptors (S1PRs) and elicits Gi-mediated signaling from one receptor subtype (the S1P1R) that is much more sustained than that elicited by the natural agonist S1P. This sustained drug response was associated with persistent
Acute GPCR-G protein signaling from endosomes
There is emerging evidence supporting the hypothesis that endosomes also function in acute G protein-mediated signaling, distinguished from sustained signaling by its onset within minutes after agonist application and its rapid reversal after agonist washout. A first hint came from study of cytoplasmic cAMP accumulation elicited by agonist-induced activation of the D1 dopamine receptor (DRD1). This Gs-mediated biochemical response was found to occur with a comparable time course as regulated
Outlook
We suggest that there is now reasonably strong evidence supporting the hypothesis that the endocytic pathway, in addition to its well-established roles in homeostatic control of GPCR number and activity, contributes to the receptor-mediated signaling response itself. Endosomes appear to support multiple mechanisms of signaling, occurring over different time scales and differing in reversibility. While much of our current understanding remains limited to indirect evidence, there is now direct
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
The authors thank present and former members of the von Zastrow lab, as well as many other colleagues and collaborators at UCSF and elsewhere, for enormous contributions of data, reagents, ideas and critical discussion. We regret not being able to acknowledge these individuals by name and highlighting only a subset of relevant research articles due to space constraints. Work in the authors’ laboratory is supported by the U.S. National Institutes of Health. R.I. is supported by a postdoctoral
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